The use of ultracapacitor packs for high-voltage, high-power energy storage applications is well known (See, for example, U.S. Pat. Nos. 6,844,704 and 6,714,391). However, the high current, low resistance characteristics of ultracapacitors present a problem during the startup (charging) phase for a completely discharged pack and the shutdown (discharging) phase of a charged pack.
Due to the low resistance of the ultracapacitors, it is usually not possible to connect the pack to a high voltage source by simply closing a high power contactor relay switch. If this is done, the initial connection of an ultracapacitor pack to a charging circuit looks like a direct short to the charging circuit and the resulting high current inrush into the ultracapacitor pack from the charging circuit can easily damage the charging circuit. One method of initially charging a completely discharged pack uses a high-power DC/DC converter so that voltage is increased slowly to limit the current flow in the circuit. Two problems with using the high-power DC/DC converter is that this component is very expensive, adding significant cost to the overall system, and is an additional component, adding complexity to the system. Furthermore, if the DC/DC converter remains in the circuit during normal operation to minimize the high voltage drop as the ultracapacitor pack discharges, the energy storage and supply cycle experiences the reduced efficiency of the two way energy path through the DC/DC converter.
At the end of an operation period, typically at the end of the day, it is desirable to discharge the ultracapacitor pack for safety, cell equalization, and increased cell life. Reducing the stand-by voltage across each cell is a means to increase the cell and pack lifetime. A passive balancing network consisting of a resistor in parallel with each cell may discharge an ultracapacitor pack but it typically requires hours for the voltage to drop to a minimum level. An active circuit to balance and/or discharge each cell may also drop the pack voltage, but it adds more cost and complexity to the ultracapacitor pack.